serial: mxc: Add common mxc_uart reg space

This patch will add common reg space for non-dm and
dm code and non-dm reg space can be accessed using
mxc_base.

This will
- get rid of __REG volatile assignments
- Make common reg_space by removing unneeded macros

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>